Thermionic and field emission devices are well known and used in a variety of applications. Field emission devices such as cathode ray tubes and field emission displays are common examples of such devices. Generally, thermionic electron emission devices operate by ejecting hot electrons over a potential barrier, while field emission devices operate by causing electrons to tunnel through a barrier. Examples of specific devices include those disclosed in U.S. Pat. Nos. 6,229,083; 6,204,595; 6,103,298; 6,064,137; 6,055,815; 6,039,471; 5,994,638; 5,984,752; 5,981,071; 5,874,039; 5,777,427; 5,722,242; 5,713,775; 5,712,488; 5,675,972; and 5,562,781, each of which is incorporated herein by reference.
The electron emission properties of thermionic devices are more highly temperature dependent than in field emission devices. An increase in temperature can dramatically affect the number of electrons which are emitted from thermionic device surfaces.
Although basically successful in many applications, thermionic devices have been less successful than field emission devices, as field emission devices generally achieve a higher current output. Despite this key advantage, most field emission devices suffer from a variety of other shortcomings that limit their potential uses, including materials limitations, versatility limitations, cost effectiveness, lifespan limitations, and efficiency limitations, among others.
A variety of different materials have been used in field emitters in an effort to remedy the above-recited shortcomings, and to achieve higher current outputs using lower energy inputs. One material that has recently become of significant interest for its physical properties is diamond. Specifically, pure diamond has a low positive electron affinity which is close to vacuum. Similarly, diamond doped with a low ionization potential element, such as cesium, has a negative electron affinity (NEA) that allows electrons held in its orbitals to be shaken therefrom with minimal energy input. However, diamond also has a high band gap that makes it an insulator and prevents electrons from moving through, or out of it. A number of attempts have been made to modify or lower the band gap, such as doping the diamond with a variety of dopants, and forming it into certain geometric configurations. While such attempts have achieved moderate success, a number of limitations on performance, efficiency, and cost, still exist. Therefore, the possible applications for field emitters remain limited to small scale, low current output applications.
A major driving force in the development of emitter technology concerns the reduction of energy required to generate luminescence as well as the resulting high heat production. Light emitting diodes (LEDs) are emitters that many have thought would be viable replacements for common illumination sources, such as fluorescent lights, and backlighting for LCD devices. The use of LEDs in such applications may not be feasible, however, due to their relatively high manufacturing cost, their difficulty in diffusing light to greater areas, and their inherent difficulty in producing natural white light.
Another potential source of illumination is that of electroluminescence (EL). Luminescence is produced in EL by applying an AC current to a luminescent material. EL devices are simpler in construction than LEDs, and are thus manufactured at a lower cost. EL devices also require less power to produce luminescence, and so generate less heat. There are at least two major obstacles, however, that preclude the use of EL devices as illumination sources. The first concerns the high operational voltages required to generate illumination. As such, the use of EL for applications such as backlighting has generated relatively dim illumination. The second obstacle relates to the rapid decay of luminosity over time.
As such, materials capable of achieving high current outputs by absorbing relatively low amounts of energy from an energy source, and which are suitable for use in illumination applications continue to be sought through ongoing research and development efforts.